[Trip planning] Through Laos Cambodia

We are two swiss students who would like to organize a trip though Laos and Cambodia during this summer, between the 15th of july and middle-august.

Medical students, we're having an internship in Vietnam, in Saigon ; afterwards, we would go along the coasts to Hanoi, were two others students would take their plane back to Switzerland.

This would be the start of our trip, and we might have some questions we hope you could answer.

1) Is it possible to BUY two motorcycles in Laos, for foreigners ? I read that you can just buy second hand bikes.
2) Is it still possible to cross the Laos-Cambodia border at Veun Kham with laos motobikes ?
3) How is the weather during end july-august ? I saw that the national 13 is a paved road, but would our trip be possible if there is monsoon ?
4) Our main path would be to join Phnom Penh from Vientiane or Luang Prabang ; what would be the minimal time necessary to archieve this road ? It would be usefull for us, so we could be able to plane our visits